1. Field of the Invention
The present invention relates to an image-forming apparatus and a process cartridge.
2. Description of the Related Art
Since electrophotographic processes allow high-speed and high-quality printing, they have been used in various electrophotographic systems such as copying machines and laser beam printers.
Recent mainstream photoreceptors used in electrophotographic systems are made of an organic photoconductive material. In terms of the structure of the photoreceptor, single-layer photoreceptors are gradually replaced with layered photoreceptors wherein a charge generating material and a charge transport material are dispersed in separate layers (charge generating and charge transport layers).
In addition, the recent trend toward improvement in the quality and speed of business processing in offices has boosted the need for faster and full-color processing of documents, which in turn has brought about improvement in the speed, quality and multi-color compatibility of image-forming apparatuss such as copying machines, printers, and facsimiles that process these documents. In response to this demand, for example, various kinds of so-called tandem color image-forming apparatuss have been developed and commercialized that have plural image-forming units respectively responsible for each of color images of black (K), yellow (Y), magenta (M), and cyan (C), and that transfer the images differing in color formed in the respective image-forming units in a superimposed manner and thus form color images on an image-receiving medium or an intermediate transfer member.
For improvement both in quality and efficiency of these color image-forming apparatuss, methods of switching image-forming modes according to the kind of image and image-receiving medium have been investigated [e.g., Japanese Patent Application Laid-Open (JP-A) No. 2003-241511]. For example, when a monochromic image is formed, the image can only be formed using a black toner, and therefore processing is likely carried out at a processing speed higher than that when forming color images. In addition, regardless of whether the image-forming apparatus is a color or monochrome machine, if the image-receiving medium is cardboard, an overhead projector (OHP) sheet or other similar medium, it is considered possible to obtain high-quality images by extending the image-forming period such that it is longer than that for usual processing.
However, an apparatus that operates under plural processing conditions (modes) that differ in the length of the period from charging to development often fails to provide images of sufficiently high quality. In other words, switching of image-forming modes inevitably leads to changes in the length of the period from charging to development, and electrophotographic photoreceptors that are compatible with such changes in usage conditions have not yet been investigated sufficiently. For example, processing conditions that elongate the period from charging to development often leads to problems of more frequent generation of image memory (images undesirably remaining on the photoreceptor after a step of eliminating charges on the photoreceptor) and images carrying a higher degree of fogging and more black spots.
Accordingly, there exists a need for an image-forming apparatus or a process cartridge that suppresses generation of fogging and black spots on output images and generation of image memory, even when the apparatus or the cartridge operates under plural processing conditions that differ in the length of the period from charging to development.